1. Field of the Invention
This invention relates to the field of video displays, and in particular concerns generation of beam deflection correction signals for cathode ray tube displays, using a stable, switched clamping circuit.
2. Description of Related Art
Certain geometrical raster distortions are associated with cathode ray tube displays. These distortions may be exacerbated in projection displays having a curved face plate CRT, with inherent magnification in the optical projection path. Curved face plate CRTs advantageously reduce projection path length and may simplify optical imaging. However, beam deflection may require especially shaped, highly stable, correction waveforms to counter raster distortion and to achieve convergence requirements imposed by large screen viewing.
Correction signals for countering pincushion distortion generally employ parabola-shaped waveforms at the horizontal and vertical scanning rates, coupled to adjust beam deflection by means of auxiliary deflection coils. A horizontal correction circuit is triggered by the retrace pulse applied to the flyback transformer and generates a generally parabolic horizontal rate driving signal coupled to the auxiliary deflection coils. Means can be included for controllably adding a tilt component to the correction signal to accommodate correction for projection beams that are not colinear. A waveform clipper is coupled to the signal generator means and clips the tilted correction signal to shape part of the correction signal.
The parabolic correction signal can be generated by triggering a ramp generator from the retrace pulse, the ramp generator having a constant current source coupled to charge a capacitor. The ramp voltage on the capacitor is integrated, for example using an operational amplifier having a capacitor in a feedback loop, to obtain a parabola signal for driving the auxiliary deflection coils. The capacitor is discharged by a signal derived from the retrace pulse, and the output of the operational amplifier is clamped, for example at -0.1 volts, for a time period between excursions of the parabola.